1. Field of the Invention
The present invention relates to improvements in electromagnetic shielding and in particular to shielding of electrical circuit assemblies and subassemblies.
2. Description of the Related Art
Isolation of circuit components from electromagnetic interference is particularly important in communications equipment, such as cable modems, cellular telephone units and related relay-station equipment having components that operate in the 10 MHz to 4 GHz frequency range. The typical RF isolation shield for electronic components in such units consists of a rectangular metallic enclosure. In order to provide adequate shielding, radiation attenuation of greater than 60 dB, typical shields are often pressure-fit with conductive gaskets, using a large number of screws or soldering.
Metallic shields may be attached and grounded by soldering; however, soldering can significantly increase the manufacturing expense of the product. Furthermore, soldering may not be reliable, especially if the shield is to be removed and reattached for repair or replacement of circuit components. After reattachment, air gaps in the seal caused by the roughness of the solder from which the shield was removed may remain. Moreover, such shields may be difficult to unsolder and the unsoldering operation itself is risky, possibly causing heat damage to electronic components or delamination of portions of the assembly. Furthermore, the use of conductive gaskets and screws for attaching such shields is expensive and labor-intensive due to the large number of screws needed to prevent EMI leakage and provide an adequate seal.
Circuits may be sensitive to electromagnetic interference for a variety of reasons, including high gain, operation at high frequencies, and the use of components and structures conducive to inductive or capacitive coupling. The same qualities also characterize circuits that emit offending high-frequency electromagnetic radiation. To attenuate an electromagnetic field in such situations, conductive material may be arranged in a complete conductive enclosure. The surface of the enclosure ideally is in close proximity to the surface of the circuit. Typical metal enclosures, however, are rectangular, the height of the enclosure being chosen to allow clearance for the tallest component on the circuit being shielded. Complex geometries are often avoided due to the relatively high cost of casting, soldering, welding, or machining such structures.
In view of the operational frequencies and character of many modem circuits, it is desirable that shielding for electromagnetic interference purposes attenuates radiation at least by 50 to 60 dB over a frequency range from 10 MHz to 4.0 GHz or more. To achieve this, the shield ideally is substantially continuous and encloses the shielded circuits closely and completely, i.e., forming a closed, highly conductive enclosure. Electromagnetic shielding for attenuating electromagnetic interference typically has a resistivity of 0.57 m.OMEGA./mm.sup.2.
It would be advantageous to provide an enclosure for providing electromagnetic shielding for a circuit assembly that is easy to manufacture.
It would also be advantageous for the shield to allow the circuit assembly to be easily and repeatably accessed by opening the shield enclosure, without compromising the integrity of the closed shield.